ALBERT

Notes: Mass resolved excitation spectra of supersonic expansion cooled mono- and diazabenzenes are reported for the low lying Rydberg states. Transitions are located for pyridine, pyrazine, and pyridazine, but not pyrimidine. The Rydberg state lifetimes of these molecules are estimated, based on a Lorentzian line shape analysis, to be ca. 500 fs. Ab initio calculations for pyrazine at the complete active space self-consistent-field (CASSCF) and CASSCF many-body second-order perturbation theory (CASSCF/MBPT2) levels show that extensive configuration interaction and dynamic electron correlation are necessary to account for the excited states of these systems. © 1995 American Institute of Physics.

Notes: Fluorescence excitation and two color mass resolved excitation spectroscopy are employed to study the D1(2A2″)←D0(2E1″) vibronic transitions of the cyclopentadienyl radical (cpd) and its van der Waals cluster with nitrogen. The radical is created by photolysis of the cyclopentadiene dimer and cooled by expansion from a supersonic nozzle. The cpd(N2)1 cluster is generated in this cooling process. Mass resolved excitation spectra of cpd are obtained for the first 1200 cm−1 of the D1←D0 transition. The excitation spectrum of cpd(N2)1 shows a complicated structure for the origin transition. With the application of hole burning spectroscopy, we are able to assign all the cluster transitions to a single isomer. The features are assigned to a 55 cm−1 out-of-plane van der Waals mode stretch and contortional (rotational) motions of the N2 molecule with respect to the cpd radical. Empirical potential energy calculations are used to predict the properties of this cluster and yield the following results: (1) the N2 molecular axis is perpendicular to the cpd fivefold axis and parallel to the plane of the cpd ring with the two molecular centers of mass lying on the fivefold ring axis; (2) the binding energy of cpd(N2)1 is 434 cm−1; and (3) the rotational motion of the N2 molecule is essentially unhindered about the cpd fivefold axis. The molecular symmetry group D5h(MS) is applied to the nonrigid cluster, and optical selection rules exclude even↔odd transitions (Δn=0, ±2, ±4,... allowed) between the different contortional levels. Tentative assignments are given to the observed contortional features based on these considerations. The barrier to internal rotation is also small in the excited state. The results for the cpd(N2)1 van der Waals cluster are compared to those for the benzene (N2)1 and benzyl radical (N2)1 clusters. © 1995 American Institute of Physics.

Notes: Cluster growth dynamics of vanadium oxide and titanium oxide clusters produced by laser ablation of vanadium and titanium metal in a He gas flow seeded with up to 2% O2 are studied by covariance mapping time-of-flight mass spectrometry. Covariance mapping enables the recognition of two different distribution components in the overall homogeneous mass spectra for both vanadium oxide and titanium oxide cluster systems. The oxygen-rich component Or shows small correlated fluctuations while the oxygen-poor component Op shows large correlated fluctuations. These two cluster distribution components are observed at low ablation laser powers and low expansion gas concentrations. Fluctuations of small vanadium oxide clusters (V2O, V2O2, and V2O3) and small titanium oxide clusters (Ti2O2 and Ti2O3) are covariance determining. The less fluctuating V2O3 and Ti2O3 clusters are "nuclei" for the oxygen-rich components Or. The more fluctuating V2O and Ti2O2 are "nuclei" for the oxygen poor components Op. Correlated fluctuations or covariances within each distribution component are constant. Covariances for the different distribution components are different. Studies of mass spectra and covariances as functions of ablation laser power and expansion gas concentration imply that V2O and Ti2O2 clusters are formed in different regions of the ablation plasma plume than V2O3 and Ti2O3. We suggest that V2O3 and Ti2O3 are formed in the hot and optically dense region near the ablated metal surface and that V2O and Ti2O2 are formed in the colder plasma region farther away from the ablated metal surface. Larger vanadium oxide and titanium oxide clusters grow from these small clusters by very specific pathways which involve only uptake of VO or VO2, and TiO2, respectively. © 1999 American Institute of Physics.

Notes: Cyclopentadienyl (cpd), methylcpd (mcpd), fluorocpd (Fcpd), and cyanocpd (CNcpd) are generated photolytically, cooled in a supersonic expansion, and clustered with nonpolar solvents. The solvents employed are Ar, N2, CH4, CF4, and C2F6. These radicals and their clusters are studied by a number of laser spectroscopic techniques: Fluorescence excitation (FE), hole burning (HB), and mass resolved excitation (MRE) spectroscopies, and excited state lifetime studies. The radical D1←D0 transition is observed for these systems: The radical to cluster spectroscopic shifts for the clusters are quite large, typically 4 to 5 times those found for stable aromatic species and other radicals. Calculations of cluster structure are carried out for these systems using parameterized potential energy functions. Cluster geometries are similar for all clusters with the solvent placed over the cpd ring and the center-of-mass of the solvent displaced toward the substituent. The calculated cluster spectroscopic shifts are in reasonable agreement with the observed ones for N2 and CF4 with all radicals, but not for C2F6 with the radicals. The Xcpd/Ar data are sacrificed to generate excited state potential parameters for these systems. CH4 is suggested to react with all but the CNcpd radical and may begin to react even with CNcpd. van der Waals vibrations are calculated for these clusters in the harmonic approximation for both D1 and D0 electronic states; calculated van der Waals vibrational energies are employed to assign major cluster vibronic features in the observed spectra. © 1999 American Institute of Physics.

Notes: Clusters of the cyanocyclopentadienyl (CNcpd) radical and several polar solvent molecules (e.g., CF2H2, CF3H, CF3Cl, CH3Cl, ROH, H2O) created in a supersonic jet expansion are studied by laser induced fluorescence and hole burning spectroscopies. Lennard-Jones–Coulomb atom–atom potential energy calculations are employed in combination with ab initio calculations to aid in the interpretation of the observed spectra and to understand the nature of the radical polar solvent solvation behavior. The calculations predict quite reasonable cluster binding energies and structures, but are less accurate in predicting van der Waals vibrational mode energies and cluster spectroscopic shifts. The limitations of the atom–atom potential energy surface model in dealing with the more subtle aspects of CNcpd–polar solvent intermolecular interactions are discussed. Some possible causes of inadequacies of the approach are presented. © 1999 American Institute of Physics.

Notes: The hydrogen abstraction reaction between cyclopentadienyl radicals [Xcpd, X=H, CH3(m), F, CN] and substituted methanes (CH4, C2H6, CH3CH2OH, CH3Cl, CH2F2, CHF3, and CH3OH) is studied for the isolated one-to-one van der Waals clusters created in a supersonic expansion. Three different types of fluorescence excitation spectra are characterized for these cluster systems: (1) sharp spectra are observed for some clusters, suggesting no cluster chemistry for either the ground or excited electronic states of Xcpd—CNcpd/CH3Cl, CH2F2, CHF3, CH3OH; (2) broad spectra are observed suggesting initiation of cluster chemistry on the excited state cluster potential energy surface—CNcpd–CH4, Fcpd–CHF2Cl, CHF3; and (3) only a greatly reduced bare radical signal is observed, but no cluster emission can be detected—cpd, mcpd/all substituted methanes, Fcpd–CH2F2, CH3Cl, CH3CH2OH, CH3OH, C2H6, and CNcpd/C2H6, CH3CH2OH. These results, taken together, suggest that the Xcpd radicals undergo an excited electronic state electrophilic hydrogen abstraction reaction with substituted methanes. The radical reactivities are in the order mcpd∼cpd〉Fcpd〉CNcpd and the substituted methane reactivities are in the order C2H6〉C2H5OH〉CH4〉CH3Cl∼CH3OH〉CH2F2〉CHF2Cl〉CHF3. All Xcpd radicals show intense, sharp spectra with CF4. This indication of an excited state Xcpd radical hydrogen abstraction reaction with substituted methanes is further explored by ab initio quantum chemistry techniques at the (7×7) CASSCF/6-31G (complete active space self-consistent field) and cc-pVDZ levels for cpd–CH4. Calculations confirm the idea that the ground state cluster has a reaction barrier (approximately +170 kJ/mol) and a positive free energy of reaction (∼80 kJ/mol). The excited cpd radical, however, can react with CH4 along a barrierless path to generate substantial hot ground product states (C5H6 and CH3). Experimental data are consistent with an Xcpd–C2H4 addition reaction, as well. © 1999 American Institute of Physics.

Notes: Neutral cluster growth and ionic cluster fragmentation are studied for toluene/water (TWn), aniline/argon (AnArn), and 4-fluorostyrene/argon (FSArn). Clusters are created in a supersonic expansion and ionized by both one-color and two-color (near threshold) resonance enhanced laser ionization. Toluene/water clusters are known to fragment subsequent to ionization by loss of water molecules or by proton transfer and loss of a benzyl radical. This system is selected to test the applicability of covariance mapping techniques to investigate the fragmentation behavior of singly charged cluster ions. To explore sensitivity of the parent ion/fragment ion correlation coefficient to cluster fragmentation, correlation coefficients are measured as a function of ionization photon energy as thresholds for the various fragmentation processes are scanned. For TW3+ parent ions, correlation coefficients correctly reflect switching between the benzyl radical loss and water loss fragmentation channels as the photon energy is increased. For T2Wn+ cluster ions, fragmentation contributes only about 20% to the correlation coefficient—the other 80% contribution is due to neutral cluster growth. The growth-dominated correlation coefficients scale approximately with the square root of the product of the two ion signal intensities and linearly with the ionization laser intensity, and therefore are not good relative measures of correlations between ions and signals of different intensities. A normalized covariance (covariance/product of signal intensities) is introduced to eliminate this dependence. The laser intensity [∼(signal product)1/2] independent component of the normalized covariance arises from ion correlation due to neutral cluster growth and the laser intensity dependent component of the normalized covariance arises from ion correlation due to cluster ion fragmentation. These findings are applied to study the cluster growth dynamics of AnArn and FSArn clusters. Covariance mapping shows that the broad intensity maxima in the mass spectrum of FSArn clusters are not caused by fragmentation but can be attributed to neutral cluster growth. The observed neutral cluster distribution appears to be a superposition of three broad, overlapping, log-normal-like distributions peaking around cluster sizes n=4, 8, 20. The difference between the overall shapes of the AnArn and FSArn mass distributions appears to be due to faster dimer and cluster growth kinetics for the FSArn cluster system. The growth kinetics for the latter two cluster systems can be fully explained and modeled by a simple closed form algebraic kinetic equation that depends on three parameters: dimer growth rate, overall cluster growth rate, and a cluster growth cross section that scales with cluster size. © 1998 American Institute of Physics.

Notes: In this work we analyze clusters between the methoxy radical (CH3O, an open-shell molecule) and the nonpolar solvents Ar, N2, CH4, and CF4. CH3O is formed through the photolysis of CH3OH vapor in a supersonic expansion of CH3OH and a solvent gas (Ar, N2, CH4, CF4) seeded in a carrier gas of He. The radical and solvent molecules are cooled to ∼15–20 K and form clusters. These clusters are probed using laser induced fluorescence (LIF) of the CH3O radical. An extensive set of calculations, including ab initio and atom–atom potential calculations and rotational contour simulations are performed for each cluster in order to elucidate the cluster structure and the nature and relative importance of the limiting types of interactions that are responsible for cluster binding. A final minimum energy structure is presented for each cluster, together with the analysis of the limiting type of interactions that generate the van der Waals binding of the cluster. © 1997 American Institute of Physics.